Problems in achieving object recognition:

1. Need to solve the "image segmentation problem": Need to distinguish between objects and their surroundings, and identify which parts belong to the and : same object. Often difficult because luminance differences may be an unreliable guide.

2. Need to identify the object, perceptually: 3. Need to identify the object, semantically: Need to recognise an object despite changes in its For familiar objects, need to access stored appearance due to changes in illumination, knowledge about their functions and meanings (e.g. viewpoint, distance, etc. "this is a chair", "I can sit on it", "this is a lecturer"). So, need to be able to generalise across views. But - may also have to distinguish between different exemplars within a category (e.g. different ).

4. Other aspects of object recognition: Need to know an object's location, relative to other objects and to ourselves.

1 : Wilbrand (1887): the case of Fraulein G.: Impairment of visual due to , aged 63. damage, which is not attributable to sensory Remained able to speak several foreign languages, impairment or gross intellectual impairment recite poetry; highly articulate. ("agnosia" lit. "without knowledge"). "When people stood at my bedside and spoke with pity of my blindness, I thought to myself: you can't really be blind because you are able to see the table-cloth over there, with the blue border, spread out on the table in the sick-room".

Mistook a dog for the doctor. Mistook a servant for the dinner-table.

The case of Fraulein G. (continued): Lissauer (1890): Topographical agnosia: unable to recognise familiar Studied G.L., who following a head , had problems recognising objects – mistook pictures for boxes, jacket places. for trousers, couldn’t recognise cutlery. Vision was intact, Object agnosia: unable to see her favourite vase, find as was his of objects. letters she had written, or her glasses. Lissauer distinguished two main types of "...at night I take any old object from the table and think; "mindblindness" (agnosia): my God, what sort of thing is this then?, and only after : protracted and repeated looking at it and palpating of it Impaired ability to consciously perceive and discriminate does it become clear to me what it is meant to be". stimuli (i.e. a perceptual deficit). Associative agnosia: Prosopagnosia (inability to recognise familiar faces) Relatively preserved ability to perceive stimuli, but including her own in a mirror. inability to interpret what was seen (i.e., a gnostic deficit). Remained capable of visual imagery.

2 Apperceptive agnosia: Apperceptive agnosia:

Typically impairments can be seen in: Can describe features of an object, but Mr. S copying, and matching: Gollin’s incomplete drawing test cannot recognise the (Warrington & James 1967). object as a whole. Cannot copy Ghent’s overlapping / embedded figures drawings. test (DeRenzi, Scotti & Spinnler 1969). Perceptual not sensory deficit (acuity Unusual views task (Warrington & Taylor 1973). etc. normal). Impaired recognition if Foreshortened photos (Humphreys & image is degraded in Riddoch 1984). any of a number of ways.

Milner & Goodale (1995): Apperceptive agnosia - pathology: D.F. : apperceptive agnosic, due to CO poisoning. Spatial frequency detection thresholds normal except for low frequencies. Unilateral right hemisphere damage. Right Flicker frequency threshold normal. inferior . Colour discrimination normal.

Object recognition grossly impaired. Cannot recognise or copy drawings of objects. Object memory intact (i.e., perceptual deficit).

3 Associative agnosia: Underlying pathology in associative agnosia: Can discriminate objects but not LF copying identify them. Usually bilateral damage. Crucial area thought to Can copy drawings. be left hemisphere. More ventral than Not a perceptual deficit. apperceptive damage. May even be able to sort objects by category. Cannot name objects and may not be able to understand their meaning. May not be able to draw objects from memory. Make structural rather than functional matches between objects (e.g. closed umbrella matched to walking stick, not open umbrella – Warrington 1982).

Modality specificity of agnosia: Modality specificity of agnosia:

Impairment in object recognition may be confined to a single sensory modality: visual agnosics are unimpaired in recognition by touch, or smell. Can have agnosia in other modalities (e.g. ). Ability to act upon and manipulate objects may be intact in agnosics – e.g. DF.

DF posting task: Cannot match using perceptual information alone, but can match orientation when asked to "post" the object. (Parallel effects now found in normal individuals).

4 “What” and “Where” systems: Category specific : (Ungerleider and Mishkin 1982, Goodale and Milner 1995): McCrae and Trolle (1956): severe impairment for Ventral stream: "what": Dorsal stream: "where": recognition of animals; unimpaired for trees, flowers, Output from V1 to V2, V4 Output from V1 to V2, V3 common objects. and IT. (Inferotemporal and MT (V5). (Posterior Hecaen & Ajuriaguerra (1956): inanimate more cortex). parietal cortex). impaired than animate. Primary function is object Primary function is spatial Warrington & Shallice (1984): JBR - animate perception + recognition. perception. impaired, but not inanimate. Warrington & McCarthy (1987) YOT - object matching impaired, but animals matching unimpaired.

Prosopagnosia - specific inability to recognise faces, but OK with non-face objects.

Implications of neuropsychological data for Elllis and Young's (1988) model of object recognition: normal perception: OBJECT

Early ideas on agnosia More modern conceptions (e.g. Farah 1990, (Lissauer 1890): Humphreys and Riddoch 1987): Initial representation IMAGE IMAGE Viewer-centred representation FEATURAL ANALYSIS

FEATURE INTEGRATION AND GROUPING Object-centred Object recognition units APPERCEPTION representation MAPPING OF PERCEPTUAL DESCRIPTIONS TO STORED STRUCTURAL DESCRIPTIONS Semantic system ASSOCIATION (must cope with unusual views, etc.)

STORED STRUCTURAL DESCRIPTIONS Name retrieval ( output lexicon) SEMANTIC KNOWLEDGE (object functions and associations) SPOKEN NAME

5 Conclusions:

Neuropsychological data suggest that (a) vision is modular - a set of processes, for different purposes. (b) location and identification are handled by largely separate, parallel processes - from through to cortex. (c) object recognition can be selectively impaired at many stages, from initial structural description (apperceptive agnosia) to linking with semantic information (associative agnosia). (d) the same is true for face recognition; there are many different causes of "prosopagnosia".

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